1. FIELD OF THE INVENTION:
The present invention relates to a request repeat method for correcting errors in data communication and more particularly, to a request repeat method suitable for mobile data communication.
2. DESCRIPTION OF THE PRIOR ART
Request repeat method for such a purpose are described, for example, in "Introduction to Data Transmission", H. Kawabata, Denki Tsushin Gijutsu News K.K., pp. 78 to 82, Apr. 20, 1970, and "Data Transmission Systems, New Edition", T. Fukushima, Sangyo Tosho K.K., pp. 216 to 218, Aug. 31, 1982. In the request repeat method described in these papers, the receiving station finds errors in data, and requests the transmitting station to repeat data transmission.
FIG. 2 illustrates a conventional transmission control procedure of a system which repeats data transmission after checking every block, in which the result of reception for every block is reported by the receiving station to the transmission station. Two kinds of codes, namely, "ACK" (acknowledged) and "NAK" (not acknowledged), are used for indicating the condition of the received data. After receiving the answer signal of the receiving station, the transmitting station transmits one data block to the receiving station and wait for the confirmation code. On the other hand, the receiving station inspects the data block by the CRC check method or the like and, if any error is found in the data block, the receiving station transmits the code NAK to the transmitting station. Then, the transmitting station, upon the reception of the code NAK, transmits the same data block again. Then, the receiving station inspects the data block again and, when the data block is correct, transmits the code ACK to the transmitting station. Thus, the condition of the data is identified by the codes for transmission of a correct data block.
The above mentioned conventional request repeat method has the following problems. A non-field state occurs for hundreds of milliseconds due to Rayleigh fading, which is specific to radio channels for mobile stations, or a hand-off operation for changing the channel from one to another in the overreaching section between radio zones in the public mobile telephone system. When the above mentioned request repeat method is employed for an ordinary frame configuration as shown in FIG. 3 in mobile data communication under such an environmental condition, a frame transmitted from the transmitting station will collectively loose the bit synchronization signal and the frame synchronization signal in the radio channel before the frame is inspected by the receiving station. Furthermore, the confirmation signals ACK and NAK are often missed to thereby interrupt the communication, and hence the request repeat system is unable to function properly.